I would not give up a Commando motor. The second paragraph in my post was for a larger engine, starting with 360/340 heads. 360/340 heads are about 8cc larger than 273 heads, about a .040 cut. Then you have to worry about intake valve hitting the block on a 273 only. I have done it, but you must measure everything and assemble carefully. But I was not advocating putting the 360/340 heads on the 273.